Best reserve missing second part of season -- that's the issue (5)
I believe the answer is:
topic
'the issue' is the definition.
(synonyms)
'best reserve missing second part of season' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'reserve missing second part of season' becomes 'ic' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'top'+'ic'='TOPIC'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for topic that I've seen before include "Theme of discourse" , "Subject (eg, of text)" , "Subject (of, eg, speech)" , "Theme (of, eg, a talk)" , "Matter for an argument" .)